1. Field of the Invention
This invention relates to a heat exchanger used for a gas containing an easily polymerizing substance and provided between a heat-exchanging gas inlet and a heat-exchanging part with a gas distributing plate.
2. Description of Related Art
The heat exchanger for transferring heat between two fluids, the one having a high temperature and the other a low temperature, is one of the chemical devices extensively used in the chemical industry. The basis of the heat exchange resides in the exchange of heat between a high temperature fluid and a low temperature fluid through the medium of a heating surface.
Generally the heat exchanger fulfills the role of exchanging heat by introducing into a heat-exchanging part thereof fluid intended to be cooled or heated. The heat-exchanging part is known in various types including the shell-and-tube type having a bundle of numerous tubes inserted in a shell, the plate type having heating plates each containing corrugated ribs or semicircular ridges superposed and clamped through the medium of a gasket after the fashion of a filter press and consequently enabled to enclose thin flow paths of a rectangular cross section therewith and allowing a high temperature fluid and a low temperature fluid to flow through the flow paths on the alternating heating plates, and the fin tube type having a heating tube provided on the inner and/or outer wall surfaces thereof with fins intended to enlarge a heating surface and heightening the effect of heat transfer, for example.
The heat exchanger is extensively utilized generally as sorted by the nature of use into (1) a heater, i.e. a heat exchanger to be used for the purpose of heating a given fluid to a required temperature without changing the phase, (2) a preheater, i.e. a heat exchanger to be used for the purpose of heating a given fluid in advance and consequently exalting the efficiency of the subsequent step, (3) a superheater, i.e. a heat exchanger to be used for the purpose of heating a given fluid till a superheated state, (4) an evaporator, i.e. a heat exchanger to be used for the purpose of vaporizing a given fluid by heating, (5) a re-boiler, i.e. a heat exchanger to be used for the purpose of causing fluid condensed in a device to be heated again till vaporization, (6) a cooler, i.e. a heat exchanger to be used for the purpose of cooling a given fluid till a required temperature, (7) a chiller, i.e. a heat exchanger to be used for the purpose of cooling a given fluid till a very low temperature below 0xc2x0 C., (8) a condenser, i.e. a heat exchanger to be used for the purpose of cooling a condensable gas till condensation and liquefaction, (9) a total condenser, i.e. a heat exchanger to be used for the purpose of thoroughly condensing a given condensable gas, and (10) a partial condenser, i.e. a heat exchanger to be used for the purpose of causing a given condensable gas to be partly condensed and liquefied and allowing the remainder thereof to be released in the gaseous state into the subsequent step, for example.
One example of the shell-and-tube type heat exchanger for one-pass operation will be described below with reference to FIG. 1. It is provided, however, that the heat-exchanging gas and/or the fluid may be led in or led out respectively through the inlet or the outlet in the opposite direction indicated in the following description, the gas may be led in or led out respectively through the fluid inlet or the fluid outlet or, by the same token, the fluid may be led in or let out respectively through the inlet or the outlet for the heat-exchanging gas, depending on the purpose or the necessity. Furthermore, the direction in which the heat exchanger is installed does not need to be limited to verticality but may be selected to suit the kind of the gas or the fluid to be handled and the purpose for which the heat exchanger is used.
With reference to FIGS. 1, 10 stands for a shell, 11 for a fluid outlet, 12 for a fluid inlet, 13 for a tube sheet, 14 for a heat-transfer tube, 15 for a baffle plate, 16 for a impingement plate, 20 and 21 each for a channel, 22 for a heat-exchanging gas inlet, and 23 for a heat-exchanging gas outlet. In FIG. 1, the part interposed between the two tube sheets (13) inside the shell corresponds to a heat-exchanging part (30).
In this heat exchanger, the gas subjected to exchange of heat is supplied through the gas inlet (22) disposed in the channel (20), then introduced into the heat-transfer tube (14), and thereafter discharged through the heat-exchanging gas outlet (23) disposed in the channel (21). The fluid, for example heating medium, is introduced through the fluid inlet (12) disposed in the shell (10), caused to exchange heat efficiently with the gas in the heat-transfer tube (14) through the medium of the heat-transfer tube (14) as guided along the flow path altered by the baffle plate (15), and led out through the fluid outlet (11). By interposing the impingement plate (16) between the fluid inlet (12) and the heat-transfer tube (14), the fluid can prevent from inducing erosion on the surfaces of the tubes which is generated by fluid contacting directly the outer wall surfaces of the bundled tubes. Generally, the cross-sectional area of the gas inlet is smaller than the area of the heat-exchanging inlet part. The reason for this difference is that when the cross-sectional area of the gas inlet is equalized with that of the heat-exchanging inlet part, it will become necessary to enlarge the gas pipe and heighten the cost.
The difference in cross-sectional area between the gas inlet and the heat-exchanging part, however, forms a cause for lowering the ratio of heat exchange because the heat-exchanging gas is supplied in a large amount to the central part of the heat exchanger and in a small amount to the peripheral part thereof. The heat-transfer tube for introducing the heat-exchanging gas, however, has never been accorded any consideration about the adoption of a device for rendering the supply of the gas uniform. Barely, the wall thickness, cross-sectional area, tube layout, and pitch of the heat-transfer tube have been studied and the shape and disposal of the baffle plate have been studied.
Particularly when the ratio of heat exchange is ununiform where the heat-exchanging gas happens to be an easily polymerizing substance-containing gas, the easily polymerizing substance tends to succumb to polymerization due to condensation. Absolutely no measure has been devised against this detriment. In the shell-and-tube type heat exchanger which has gas pipes drawn in from the top of a distillation column, for example, the vapor abounding in a low boiling component and ascending to the top of the distillation column is cooled and condensed inside the heat-transfer tubes. This vapor tends to succumb to polymerization inside the heat exchanger when the substance subjected to distillation happens to be such an easily polymerizing compound as acrylic acid. For, the acrylic acid gas which has been obtained prevalently by the catalytic gas phase oxidation of propylene, for example, contains such impurities as water, acetic acid, and acrolein and tends to induce polymerization of acrylic acid easily. This polymerization cannot be prevented fully satisfactorily even by adding to the process a varying polymerization inhibitor such as, for example, phenothiazine, hydroquinone, methoquinone, cresol, phenol, or t-butyl catechol. Since such a polymerization inhibitor is a high boiling substance, the temperature conditions capable of gasifying the easily polymerizing substance fail to bring though incorporation of the polymerization inhibitor in the gas to be formed under such conditions. It follows that the slender heat-transfer tubes are liable to induce polymerization in their interiors and suffer deposition of a polymer on the inner walls thereof because the composition itself is in a very easily polymerizing state and, moreover, the polymerization inhibitor does not effectively discharge its own function in a gas.
The problems regarding the uniformity of the ratio of heat exchange, the distribution of the heat-exchanging gas, and the generation of a polymer by the easily polymerizing substance which are encountered by the heat exchanger are not limited to the shell-and-tube type heat exchanger mentioned above but are entailed by the fin tube type heat exchanger and the plate type heat exchanger as well.
Absolutely no study has ever been made as to the distribution of the heat-exchanging gas. Particularly in the heat exchange of an easily polymerizing gas, the problems such as the generation of a polymer in portion of the heat-exchanging part due to the degradation of the heat-transfer efficiency resulting from ununiform supply of the gas and also due to the concentration of the feed gas, the forced suspension of the entire system, and the decline of the heat-transfer efficiency resulting from the deposition of a polymer on the heat-transfer surface remain yet to be solved.
The present inventor has performed an elaborate study on the structure of a heat exchanger and has consequently found that the provision of a gas distributing plate between the heat-exchanging part and the heat-exchanging gas inlet allows the gas to be uniformly supplied to the heat-exchanging part and that the generation of the polymer can be effectively depressed by the disposition of the gas distributing plate. This invention has been perfected as a result.
To be specific, this invention is aimed at providing the following intellectual achievement.
A heat exchanger for an easily polymerizing substance-containing gas provided with a shell possessed of a heat-exchanging gas inlet and a heat-exchanging gas outlet and a heat-exchanging part adapted to circulate fluid introduced from outside the shell between the gas inlet and the gas outlet, which heat exchanger is characterized by being provided with a gas distributing plate between the gas inlet and the heat-exchanging part.
According to this invention, since the heat-exchanging gas containing an easily polymerizing substance is uniformly distributed in the heat-exchanging part inside the heat exchanger, the generation of a polymer which possibly occurs when the gas is ununiformly supplied to the heat-exchanging part can be repressed. When an easily polymerizing substance-containing gas contacts a structure where the distribution of a gas is not uniform, the gas touching the surface of the structure is caused to condense and stagnate and consequently give rise to a polymer. In the heat exchanger of this invention, it is made possible by effecting uniform distribution of the gas in the heat-exchanging part to repress the stagnation of the gas subsequent to the condensation and prevent the easily polymerizing substance from polymerizing.